Coiled coils are simple structures consisting of two or more amphipathic helices that wrap around each other with a slight supercoil. We have tested the principles that guide folding of coiled coils by designing three peptides that preferentially associate with each other to form a parallel heterotrimeric coiled coil. We propose solving the structure by measuring anomalously scattering data from the PtCl2 derivative. The quality of data obtained from a MAD experiment would be particularly good in this case since the derivatized crystals diffract to high resolution. The crystal structure of the hetrotrimer will reveal the conformational details of a designed protein.